Route unknown error to parent exception's handler

Co-authored-by: Johannes Müller <straightshoota@gmail.com>

spec/exception_handler_spec.cr

@@ -129,6 +129,29 @@ describe "Kemal::ExceptionHandler" do
     response.body.should eq "A custom exception of CustomExceptionType has occurred"
   end
 
+  it "renders custom error for a child of a custom exception" do
+    error CustomExceptionType do |env, error|
+      "A custom exception of #{error.class} has occurred"
+    end
+
+    get "/" do
+      raise ChildCustomExceptionType.new
+    end
+
+    request = HTTP::Request.new("GET", "/")
+    io = IO::Memory.new
+    response = HTTP::Server::Response.new(io)
+    context = HTTP::Server::Context.new(request, response)
+    Kemal::ExceptionHandler::INSTANCE.next = Kemal::RouteHandler::INSTANCE
+    Kemal::ExceptionHandler::INSTANCE.call(context)
+    response.close
+    io.rewind
+    response = HTTP::Client::Response.from_io(io, decompress: false)
+    response.status_code.should eq 500
+    response.headers["Content-Type"].should eq "text/html"
+    response.body.should eq "A custom exception of ChildCustomExceptionType has occurred"
+  end
+
   it "overrides the content type for filters" do
     before_get do |env|
       env.response.content_type = "application/json"

spec/spec_helper.cr

@@ -29,6 +29,9 @@ end
 class CustomExceptionType < Exception
 end
 
+class ChildCustomExceptionType < CustomExceptionType
+end
+
 add_context_storage_type(TestContextStorageType)
 add_context_storage_type(AnotherContextStorageType)
 

src/kemal/exception_handler.cr

@@ -13,20 +13,38 @@ module Kemal
     rescue ex : Exception
       # Use error handler defined for the current exception if it exists
       return call_exception_with_exception(context, ex, 500) if Kemal.config.error_handlers.has_key?(ex.class)
+
+      # Use error handler for an ancestor of the current exception if it exists
+      Kemal.config.error_handlers.each_key do |key|
+        if key.is_a? Exception.class && ex.class <= key
+          return call_exception_with_exception(context, ex, 500, override_handler_used: key)
+        end
+      end
+
       log("Exception: #{ex.inspect_with_backtrace}")
       # Else use generic 500 handler if defined
       return call_exception_with_status_code(context, ex, 500) if Kemal.config.error_handlers.has_key?(500)
       verbosity = Kemal.config.env == "production" ? false : true
       render_500(context, ex, verbosity)
     end
 
-    private def call_exception_with_exception(context : HTTP::Server::Context, exception : Exception, status_code : Int32 = 500)
+    # Calls the defined error handler for the given exception if it exists
+    #
+    # By default it tries to use the handler that is defined for the given exception. However, another
+    # handler can be used via the `override_handler_used` parameter.
+    private def call_exception_with_exception(context : HTTP::Server::Context, exception : Exception, status_code : Int32 = 500, override_handler_used : (Exception.class)? = nil)
       return if context.response.closed?
 
-      if !Kemal.config.error_handlers.empty? && Kemal.config.error_handlers.has_key?(exception.class)
+      if !override_handler_used
+        handler_to_use = exception.class
+      else
+        handler_to_use = override_handler_used
+      end
+
+      if !Kemal.config.error_handlers.empty? && Kemal.config.error_handlers.has_key?(handler_to_use)
         context.response.content_type = "text/html" unless context.response.headers.has_key?("Content-Type")
         context.response.status_code = status_code
-        context.response.print Kemal.config.error_handlers[exception.class].call(context, exception)
+        context.response.print Kemal.config.error_handlers[handler_to_use].call(context, exception)
         context
       end
     end